More than 30 leaders are in the Colombian port city for the summit, which will see talks on economic policy and trade.

The BBC's Arturo Wallace in the capital Bogota says that, even before the incident with the agents, the summit seemed likely to be an awkward one for the US government, with debates on the legalisation of drugs and sanctions against Cuba promised.

Ecuadorean President Rafael Correa is boycotting the summit because of Cuba's exclusion, and the attendance of Venezuelan President Hugo Chavez is in doubt.

Earlier, Colombian police said two small explosions took place in Cartagena, but caused no casualties or damage.

Those explosions followed at least one other blast in Bogota near the US embassy, officials said.